The present invention relates generally to methods of producing compositions and, more particularly, to a method of producing a blended cement having enhanced cementitious properties.
Portland cement is widely used in construction applications. Portland cement is composed primarily of calcium silicate and calcium aluminate minerals capable of reacting with water to form a dense, solid paste. The reaction products of portland cement are calcium silicate hydrate, ettringite in the poorly-crystalizine phase, and calcium hydroxide occupying a majority of the pore spaces. Although portland cement is a widely utilized construction material, it is not without drawbacks.
To illustrate, portland cement concrete has a relatively high permeability to water. Accordingly, portland cement concrete structures are susceptible to deterioration if water containing sulfates is allowed to enter the pore structure. Such deterioration may include damage to the concrete as well as corrosion of metal reinforcing bars. In addition to high permeability, portland cement releases heat upon hydration that may lead to thermal cracking in mass concrete structures. Also, the burning process required for the production of portland cement clinker causes considerable emissions of NOX, SOX and CO2, often referred to as greenhouse gases.
In an effort to improve the performance of portland cement concrete, supplementary cementing materials (SCMs), such as fly ash, may be combined with portland cement to produce a blended cement. Fly ash is typically a by-product of burning coal, generated during the production of electricity at coal-fired power plants.
Fly ash typically contains about 85% glassy, amorphous components. ASTM C 618-85 (xe2x80x9cStandard Specification for Fly Ash and Raw Calcined natural Pozzolan for Use as a Mineral Admixture in Portland Cement Concrete, pp385-388 (1985)) has classified fly ash into two classes, Class C and Class F. Class F fly ash typically contains more than 70% by weight of silica, alumina, and ferric oxides, while Class C typically contains between 70% and 50%. Class C fly ash is usually high in calcium and produced as a by-product of the combustion of lignite or sub-bituminous coal.
Generally, cements containing SCMs exhibit improved later strength in concrete (lower cement factors for equal 28 day strength), improved sulfate resistance, lower heat of hydration in mass concrete, improved durability, and reduced resistance to alkali-silica reactivity (ASR). Also, the use of SCMs in the production of concrete is an environmentally friendly option. Each pound of portland cement clinker that is replaced by a SCM does not require the aforementioned burning process, thus sparing the environment from such emissions.
Although blended cements are preferable to portland cement for a variety of applications, it is often not economically viable for a cement manufacturer to produce a blended cement for sale to the concrete producer. Fly ash, especially class C fly ash, is inexpensive and easy to acquire. In short, the concrete producer is not likely to pay portland cement prices for a blended cement. A blended cement, by definition, replaces a portion of the portland cement with relatively inexpensive fly ash. Thus, from an economic standpoint, the concrete producer is more likely to buy portland cement and a supplementary cementing material(s) separately and mix them at the concrete mixer. In doing so, the concrete producer maintains control of the concrete""s mixture ratio. By adjusting the amount of SCMs at the concrete mixer, the concrete producer is able to vary concrete characteristics such as compressive strength, setting time, sulfate resistance, heat resistance, workability, and ASR resistance.
Recent studies by the Texas Department of Transportation (TxDOT) have shown that blended cements containing a mixture of portland cement and class C fly ash are often ineffective in preventing ASR and, in some cases, contribute to concrete deterioration. As a result, TxDOT has virtually banned the use of class C fly ash for State of Texas projects that require ASR and sulfate resistance. Specifically, class C fly ash may only be used in these applications if testing confirms that the resulting concrete conforms to ASR and sulfate resistance standards as set forth in ASTM C-595 and ASTM C-1157.
There remains a need for a blended cement capable of exhibiting ASR and sulfate resistance properties while still allowing the addition of class C fly ash at the concrete mixer.
Accordingly, the present invention provides a method of producing a blended cement capable of meeting ASTM C-595 and C-1157 standards. The blended cement of the present invention contains portland cement clinker, calcium sulfate and volcanic ash that has been subjected to heat in the range of between 800xc2x0 F. and 1300xc2x0 F. and contains coarse particles which must be ground for activation.
The portland cement clinker, calcium sulfate and volcanic ash utilized by the present invention are interground to an ultimate blaine fineness of between 350 m2/kg and 500 m2/kg. The resulting blended cement of the present invention is capable of subsequent admixing with class C fly ash to produce synergistic effects. Specifically, the addition of class C fly ash to the blended cement containing interground volcanic ash, portland cement clinker, and calcium sulfate is capable of producing an ultimate blended cement exhibiting improved sulfate resistance, improved workability, reduced permeability with regard to chlorides and sulfates, improved resistance to ASR and elimination of delayed ettringite formation.